This invention relates to a resin composition and processes for producing a multilayer printed circuit board using said resin composition.
Usual multilayer printed circuit boards are produced by piling a prepreg, which is obtained by impregnating glass cloth with an epoxy resin and semi-curing the resin, together with a copper foil on an insulating substrate having interlayer circuits thereon, laminating them into an integrated structure by pressing with heating, drilling holes for inter-laminer connection, conducting electroless plating on the inner walls of through-holes and copper foil surface, if necessary, conducting electrolytic plating so as to make circuit conductors in necessary thickness, removing selectively unnecessary copper, and forming circuit conductors.
With recent tendency of miniaturization, light weight and multi-functionality of electronic devices, an increase of the degree of integration of large scale integrated circuits (LSI) and miniaturization of chip parts proceed together with an increase of connecting pins and miniaturization of parts. Thus, in order to mount these parts, wiring in multilayer printed circuit boards becomes finer and finer. But the reduction of the width of wiring has naturally a limit technically. Today, the minimum wiring width for mass production is about 100 .mu.m. Further, the size of through-holes, which occupies a considerably large area compared with the wiring width, becomes an obstacle for improving wiring density. These through-holes are drilled mechanically in general. Since it is difficult to make the diameter of a drill smaller from the viewpoint of maintaining mechanical strength, it is difficult to make the areas of through-holes smaller.
Recently, it is reported various processes for forming through-holes without using a drill. For example, there are reported, first, hole fabrication using a laser, second, a process for forming via holes selectively, and third, a process for selectively removing only an insulating material by etching.
According to the first process of using a laser, the diameter of through-holes can be lessened, but the laser per se is expensive, and it requires a step for removing molten glass, resins, etc. after hole-fabrication, and thus not efficient.
According to the second process of selective via hole formation, a multilayer printed circuit board is produced by forming holes previously in a prepreg or a thin adhesive sheet, laminating the prepreg or the like on a circuit substrate, electrically connecting inner walls of the holes to outer conductors using an electroconducting substance, conducting circuit processing, and repeating these steps for forming multilayers. But since the resin of prepreg or the adhesive of the adhesive sheet has fluidity, when holes having a small diameter are formed, the resin or the adhesive is forced to ooze into the interior of the holes at the time of lamination, resulting in making the hole diameter smaller.
According to the third process, the problem mentioned above can be solved. For example, as disclosed in Japanese Patent Examined Publication (JP-B) No. 42-24109, an insulating adhesive is attached to a metallic foil, followed by selective removal of a part of the metallic foil, wherein the removed portion can be corroded with a suitable corrosive. Thus, small holes which can not be formed by drilling can be fabricated. Further, it is not necessary to use an expensive special device such as a laser. But according to said JP-B, a thin film of polyester is used as an insulating material and concentrated sulfuric acid is used for corrosion. But, the polyester film is not used in printed circuit boards now. Thus, the use of polyester film is not practical.
The use of an alkali-soluble resin as an insulating material in place of the polyester is disclosed in Japanese Patent Unexamined Publication (JP-A) No. 5-259649. According to said JP-A, a multilayer printed circuit board is produced by forming an insulating layer on an inner layer circuit substrate using an uncured alkali-soluble resin, laminating a copper foil thereon, forming an etching resist except for positions of holes to be formed, removing the exposed copper foil not covered with the resist by etching using a chemical etching solution, further removing the exposed alkali-soluble resin by etching using an alkaline solution to form via holes, curing the alkali-soluble resin, and metallizing the inner portion by electroless plating or the like for electrical connection. But according to the process of said JP-A, there are the following problems:
(i) Since the alkali-soluble resin is used, it is not possible to use an alkaline solution in other steps. PA0 (ii) When the alkali-soluble resin is cured, the resin is fluidized to make the hole diameter smaller.